The process of dark adaptation is the process by which sensitivity of the visual system, namely the eye and the brain to light increases when an individual is placed in darkness. The ability of one or both eyes to adapt to darkness may be impaired by a number of causes, for example nutritional deficiencies, diseases and inherited abnormalities. When the ability of the eye of an individual to adapt to darkness is impaired, either the speed or the extent or both at which the eye can adapt to darkness may be adversely affected. Attempts in the past have been made to provide apparatus for testing dark adaptation of the eye of an individual; however, all such apparatus suffer from disadvantages. In particular, it has been found that such known apparatus are unsatisfactory for use in testing dark adaptation of a child. Further, most tend to be relatively cumbersome and/or relatively expensive and in general most require a relatively skilled operator to measure dark adaptation.
A typical example of apparatus for testing dark adaptation is disclosed in U.S. Pat. No. 3,936,162. This apparatus comprises a face mask having an eye piece for fitting on the subject. A pair of test stimuli provided by two light emitting diodes are mounted in the eye piece so that, in use, the light emitting diodes are in alignment with the optical axis of the subject's eyes. A control circuit provided in a remotely mounted cabinet controls the level of luminance of the diodes in a time ordered sequence. The level of light intensity of the diodes sequentially increases from commencement of the test until it reaches a maximum and then is returned to its minimum value and it increases again sequentially in a time ordered sequence to the maximum and so on. On a subject perceiving the light, the subject operates a switch which stops the sequence. The level of luminance of the diodes when it became visible to the subject is displayed. The person carrying out the test manually times the test and on a level of luminance being displayed records the time and level of luminance. The test is continued in this fashion and the ability of the subject to adapt to darkness is determined from the recorded results.
However, this device is particularly unsuitable for use with children, since a child could indicate that the light emitting diodes were visible to it when in fact they weren't. This obviously would give incorrect results. A further problem with this apparatus is that by virtue of the fact that the level of luminance of the diodes continuously alters in a time ordered sequence, the apparatus tends to give relatively inaccurate results, whether in a child or adult subject. Furthermore, by virtue of the fact that the person carrying out the test must determine the time at which the light emitting diodes become visible to the subject an accurate determination of time is relatively difficult to obtain. Additionally, a further problem with this apparatus is that the area of the retina being tested is limited to the central region. This is because the light emitting diodes are located on the optical axis of the eye, and in close proximity to the eye.
Another apparatus for testing dark adaptation as well as other parameters of the eye is described in British Patent Specification No. 2,114,406. This apparatus comprises a cathode ray tube which displays to a subject a fixation point anywhere in the field of the tube screen and a test stimulus which moves on the screen. The brightness of the test stimulus is adjustable over a range of 10,000 to 1. The background brightness of the screen is also adjustable. By moving the test stimulus and altering the brightness and determining the time at which the test stimulus becomes visible the dark adaptation of the eye of the subject may be determined. However, this device also suffers from the serious disadvantage that it is unsuitable for use with a child since the child could indicate that the test stimulus was visible to it when, in fact, it wasn't, thus giving spurious and inaccurate results. Furthermore, a particular disadvantage of this device is that it tends to be relatively complex, difficult to operate and relatively expensive to produce.
There is therefore a need for apparatus for testing dark adaptation of the eye which overcomes the problems of known apparatus. The present invention is directed towards providing such apparatus.